Omid,
You could always install Win 7 RC that would upgrade you to the top of the class.
I would upgrade to Win 7 when final version come, not beta/RC
The RC is extremely stable. I'm currently running it on 1 desktop and 1 laptop. I have not encoutered any problems and, it's free for the next year.
Here's an interesting
WINDOWS 7 TWEAK GUIDE.
I don't agree with their recommendation to turn off UAC and System Restore in order to gain max system response but, that's a users choice.
I would rather have a system that's a little slower but a lot safer. IMHO